1. Field of the Invention
The present invention relates to electronic voltage regulators and, more particularly, to electronic voltage regulators where the voltage source is of a low voltage value.
As certain portable devices become smaller and smaller, miniaturization of the components therein must necessarily follow. This often includes the battery used to provide electrical energy to the portable device. Such small batteries having a satisfactorily long life often have relatively low voltages in the one to two volts range, for example.
Nevertheless, the miniaturized electronic circuits which are to be operated from such a battery must many times meet very demanding standards. For instance, such a battery may supply power to amplifiers having a relatively large gain thereacross, often making them susceptible to any noise introduced though the power supply. The battery may also have to provide current to difficult loads such as significantly inductive loads. This is compounded by such batteries often having internal impedances of several ohms to perhaps twenty-five ohms or more. As a result, there can be voltage disturbances on the battery supply lines which may be tens of millivolts in magnitude or more. In some instances, the situation can be made worse by occurrance of regeneration through the circuits in the system.
Such circumstances usually require the use of a voltage regulator between such a battery power supply and the electronic circuits, or at least do so in many systems or parts of systems. Such a voltage regulator must typically be capable of providing a very stable voltage output. Further, the regulator must provide such a stable voltage output even as the battery, in its later stages of life, has an output voltage which comes closer and closer to the desired regulator output voltage in value. Such regulator performance is desirable because the useful life of the battery is thereby extended if it can be used even though its voltage has come quite close to the needed output voltage of the regulator. Of course, the current drain caused by the regulator should be minimal to also lengthen the life of the battery.
The use of electronic series regulators with a series-pass transistor as the primary element controlling the flow of current to the regulator output presents difficulties because of device threshold limits and because the device gain varies with the voltage drop thereacross. The device gain drops as the voltage thereacross drops, making it difficult to control sharp voltage disturbances at the regulator output in the later stages of battery life. Such disturbances could be reduced by use of a capacitor of sufficient size across the regulator output, but such a capacitor cannot be formed in an integrated circuit. Such a capacitor, however, will be an undesirable solution in terms of the space required for the capacitor and its cost. A shunt regulator with a parallel-pass transistor is another possibility, were it not for the current drain such a regulator entails at least at some power supply voltages. Thus, a regulator is desired that operates satisfactorily in these circumstances.